Slash and Burn Impacts on a Costa Rican Wet Forest Site

Impacts of felling, mulching, and burning on budgets of C, N, S, P, K, Ca, and Mg; rates of CO2 evolution from the soil; soil seed storage; and plant growth were evaluated. The felled tropical evergreen forest was 8—9 yr old, interspersed with patches of 70—yr—old forest and had a leaf area index of 6 and aboveground biomass of 5.2 kg/m2. Harvest of the largest trees removed 18% of the S, and >10% of all other elements except N. During the 11—wk mulching and drying periods, 33% of the K and 13% of the P disappeared, but losses of other elements were <6%. Burn temperatures were >620°C in surface fuels, but soil temperatures were seldom >100°C at 1 cm or >50°C at 2 cm. The burn volatilized 1600 g/m2 C, 49 g/m2, and 13 g/m2 S. Postburn wind and water erosion of ash, plus leaching, removed 34 g/m2 N, 20 g/m2 K, 1 g/m2 P, 39 g/m2 Ca, and 7 g/m2 Mg, but insignificant amounts of C and S. After the burn and onset of the rains, 57% of the initial amount of N and 39% of the initial C still remained because of conservation of the organic—rich upper 3 cm of soil. Soil CO2 evolution was greater from beneath the 11—wk—old slash (3.6 gC°m—2°d—1), probably because the slash conserved soil moisture better than the actively transpiring forest. After the burn both the burned field and forest soil evolved CO2 at °4.5 gC°m—2°d—1. At this rate, 154 d of decomposition and respiration would release as much C into the atmosphere as did the burn. Soil seed storage dropped from °8000 seeds/m2 (67 species) in the forest, to 6000 seeds/m2 (51 species) after 11 wk of mulching, to 3000 seeds/m2 (37 species) after the burn. The seeds not killed by the burn, the survival of mycorrhizal fungi, and the release of nutrients resulted in vigorous and diverse postburn regrowth.